Copy Linked List to Another List? - c++

Question

I need help in linked list, to make a function for copy a list to another list
I code with Visual Studio 2012. Here is my code and the error that I get:
ERROR:
Unhandled exception at 0x0111544F in linked list.exe: 0xC0000005:
Access violation writing location 0xCDCDCDCD.
Could someone please tell me what's the mistake and how to fix it?
#include <iostream>
using namespace std;
struct node
{
int info;
node *next;
};
void PrintList (node *C)
{
node *P;
cout<<"Node content = (";
P = C;
while(P!=NULL)
{
cout<<P->info;
P = P->next;
if(P != NULL)
cout<<",";
}
cout<<")"<<endl;
}
void copylist (node *Y, node **Z)
{
node *P, *Q;
int temp;
if (Y==NULL)
{
cout<<"the list is empty"<<endl;
*Z=NULL;
}
else
{
P=Y;
Q=*Z;
while (P->next!=NULL)
{
temp=P->info;
Q->info=temp;
P=P->next;
Q=Q->next;
}
}
}
void insertnode (int *A, node **Q)
{
node *N, *P;
N=new node;
N->info=*A;
N->next=NULL;
if(*Q==NULL)
*Q=N;
else
{
P=*Q;
while (P->next!=NULL)
P=P->next;
P->next=N;
}
}
int main ()
{
node *z;
node *duplicate;
z=NULL;
duplicate=new node;
int e;
int i;
int *temp;
temp=new int;
cout<<"the number of element : ";
cin>>e;
cout<<" LIST Z CONTENT : " <<endl;
PrintList (z);
for (i=0;i<e;i++)
{
cin>>*temp;
insertnode(temp, &z);
}
copylist(z,&duplicate);
cout<<" DUPLICATE LIST CONTENT : "<<endl;
PrintList(duplicate);
}

Answer 1

Please note that in main function, you are trying to print an empty list with the following code:
cout<<" LIST Z CONTENT : " <<endl;
PrintList (z);
So, you may want to modify your print function as:
void PrintList(node *C)
{
if (C == NULL) return; // return if the list is empty or print out a message
node *P = C;
cout << "Node content = (";
while (P != NULL)
{
cout << P->info;
P = P->next;
if (P != NULL)
cout << ",";
}
cout << ")" << endl;
}
And in your copy function, you forgot to create new node and keep the link to it during the copy process. Here is the code that would work:
void copylist(node *Y, node **Z)
{
if (Y == NULL)
{
cout << "the list is empty" << endl;
*Z = NULL;
}
else
{
node*P = Y;
node *Q = NULL;
while (P != NULL)
{
node* newNode = new node; // first create a node (allocate memory)
newNode->info = P->info; // then fill the info
newNode->next = NULL;
if (Q == NULL)
{
Q = newNode;
*Z = Q;
}
else
{
Q->next = newNode;
Q = Q->next;
}
P = P->next;
}
}
}
Lastly, your main function does not return anything!! You may want to add
return 0;
at the end.
Try replacing your main function with:
int main()
{
node *z = NULL;
node *duplicate = NULL;
int e;
cout << "The number of elements : ";
cin >> e;
cout << "enter elements with a space between them : ";
while (e--)
{
int temp;
cin >> temp;
insertnode(temp, &z);
}
cout << " LIST Z CONTENT : " << endl;
PrintList(z);
copylist(z, &duplicate);
cout << " DUPLICATE LIST CONTENT : " << endl;
PrintList(duplicate);
cin >> e;
return 0;
}
Alternatively, for copying a list to another, you can call the insertnode function in a loop for adding all the data of the original list to the new list in a loop. This will make your copy function even shorter.
For more information about Linked Lists:
http://en.wikipedia.org/wiki/Linked_list
Happy coding!

Answer 2

You can't copy a list without allocating memory.
You have an access violation cause you're not creating new nodes in your destination list when you copy, but rely on whatever (corrupt pointer) is in next when you use a node.
Other than that you have a small memory leak.
Note the lines with comments:
void copylist (node *Y, node **Z)
{
node *P, *Q;
if (Y==NULL)
{
cout<<"the list is empty"<<endl;
//*Z=NULL; // memory leak! lost pointer of caller!
}
else
{
P=Y;
Q=*Z;
while (P->next!=NULL)
{
Q->info = P->info;
P=P->next;
Q->next = new node(); // missing allocation!
Q = Q->next;
}
Q->next = null; // close new list.
}
}